text: Most biological macromolecules contain few or no halogen atoms.  But when molecules do contain halogens, halogen bonds are often essential to understanding molecular conformation.  Computational studies suggest that known halogenated nucleobases form halogen bonds with oxygen, nitrogen, or sulfur in vitro.  Interestingly, oxygen atoms typically do not attract halogens with their lone pairs, but rather the π electrons in the carbonyl or amide group.
  Halogen bonding can be significant in drug design as well.  For example, inhibitor IDD 594 binds to human aldose reductase through a bromine halogen bond, as shown in the figure.  The molecules fail to bind to each other if similar aldehyde reductase replaces the enzyme, or chlorine replaces the drug halogen, because the variant geometries inhibit the halogen bond.